Prior-art devices used in panoramic X-ray imaging for moving various functional elements, such as different supporting arms, an X-ray source and a film cassette or an image detector unit, include e.g. stepping motors. In this type of imaging, all movements of the functional elements have to be precise, accurate and smooth. An extremely critical movement is that of a light film cassette or image detector unit, which must be void of any vibrations as these would impair the imaging result. A serious problem associated with stepping motor drives is that, to achieve a sufficiently smooth and vibration-free motion, it has been necessary to use a gear solution of an extremely accurate and backlash-free design. The so-called zero-backlash type gear often used in this connection is because of its construction very expensive and requires adjustment and periodic maintenance, or replacing of it with new gears as a consequence of wear.
A prior-art panoramic X-ray apparatus is represented in U.S. Pat. No. 4,683,581 (FI patent no. 88671). The invention described in this publication mainly relates to the geometry of the movements of functional elements, and it proposes the use of stepping motors as a means of producing said movements. The specification presents no details as to how the stepping motor drive is to be implemented. In a commercial apparatus corresponding to this patent the functional elements are moved using stepping motors through gears.
Another prior-art panoramic X-ray apparatus that uses stepping motors is represented in U.S. Pat. No. 5,903,126 (FI patent 98011). This specification is mainly concerned with the control of stepping motors and the problems resulting from changes in the rotational speed of the motor. When the rotational speed of a stepping motor is changed, its control frequency changes in a notch-by-notch manner, and those momentary frequency changes are extensive at high rotational speeds of stepping motors. The solution according to this patent discloses a circuit arrangement by means of which the changes of speed can be performed in a stepless manner in the analog domain. However, this solution involves the drawback that, as the stepping motor works in a stepwise manner and at relatively high revolutions, it has to be operated in conjunction with a reduction gear, which is an expensive component requiring maintenance. Further drawbacks with this invention are the complexity of the circuit solution and the large number of components needed in it. Further, the number of components is multiplied when several stepping motors of the panoramic X-ray imaging apparatus are to be controlled.
A further common problem in the use of stepping motors has been the noise. Due to the solutions used to control and operate stepping motors, prior-art applications often produce an audible high-frequency whistling sound that does not naturally fit into a medical imaging situation and environment.